Method for the two-stage epitaxial growth of iii&#39; v semiconductor compounds

ABSTRACT

An improved furnace boat method has been developed for the epitaxial solution growth of III-V compounds. The boat comprises one or more closed compartments wherein the growth solution or solutions are located, in combination with a sliding substrate support member at the base of the solution compartments, such that the substrate wafer can be moved quickly and easily into contact with, or out of contact with, the growth solution. The technique permits optimum surface protection of the substrate prior to immersion, removal of the slice at any time increment of the growth and/or cooling cycle, and will ensure complete removal of gallium solution from the grown surface without damage to the surface.

United States Patent 1191 1111 Stone et al. [45] Dec. 9, 1975 [5 METHODFOR THE TWO-STAGE 3.565.702 2/1971 Nelson 148/172 EPITAXIAL GROWTH OF v3,759.75) 9/I973 Solomon H 348/1171 SEMICONDUCTOR COR/POUNDS 1770518 1H1973 Rosztoczy....... I48/ I 71 [75] Inventors: Louis Earl Stone,Richardson, Tex.; Primary Emminer G Ozaki Romano Mexico City Attorney.Agent, or Firm-Hal Levine; James T. Comfort; Gary C. Honeycutt (73]Assignee: Texas Instruments Incorporated,

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a 57 ABSTRACT 22 F] d: l 7 l Juy 24 19 3 An improved furnace boat methodhas been deveL [21] Appl. No: 382,254 oped for the epitaxial solutiongrowth of Ill-V com- Related U S Applicmion Dam pounds. The boatcomprises one or more closed com- 62 g partments wherein the growthsolution or solutions are I 1 321 22? MW I971 located, in combinationwith a sliding substrate support member at the base of the solutioncompart- V ments, such that the substrate water can be moved [52!148/171 [48/ quickly and easily into contact with, or out of contact IntCl 2 H0"; 7 38 with, the growth solution. The technique permits opti- 8]Field a 1473A mum surface protection of the substrate prior to im rc 1mersion, removal of the slice at any time increment of I the growthand/or cooling cycle. and will ensure complete removal of galliumsolution from the grown sur (56] V Rekrences Cited face without damageto the surface.

UNI FED STATES PATENTS 3.551.219 12/1970 Punish et al. 148/171 5 Claims2 Drawing Figures U.S. Patent Dec. 9, 1975 3,925,117

METHOD FOR THE TWO-STAGE EPITAXIAL GROWTH OF Ill-V SEMICONDUCTORCOMPOUNDS This application is a division of application Ser. No. M7998filed May 28, l97l, now U.S. Pat. No. 3,747,562.

This invention relates generally to the epitaxial solution growth ofcrystals, and more particularly to the growth of semiconductor crystals,including IIIV compound semiconductor crystals, for example. An improvedfurnace boat is provided which includes a sliding substrate supportmember at the base of one or more solution compartments, including meansfor complete removal of gallium solution from the grown sur face upondisplacing the substrate from the growth solution or solutions.

Various techniques for the epitaxial solution growth of semiconductorcrystals are known to the industry. For example, the tipping" method hasbeen used, wherein a suitable substrate, such as a gallium arsenideslice, is held in one end of a container wherein a suitable growthsolution is kept separate from the substrate simply by tilting thecontainer. When the container and its contents are heated to a suitabletemperature, the entire furnace and container are moved to a levelposition thereby immersing the substrate slice. A cooling program isthen carried out, during which interval super-saturation produces sourcematerial for epitaxial growth. After suitable growth, the furnace isagain tilted to its initial position, which is intended to de cant" thesolution from the surface of the slice. Actually, however, suchdecanting usually fails to occur, leaving a substantial accumulation ofliquid on the surface of the substrate slice. Upon cooling to roomtemperature, additional, undesired, irregular growth occurs. Moreover,excessive decomposition of the sub strate surface frequently occursduring the initial heating step before immersion.

It is also known simply to dip a vertically held substrate into acrucible containing a suitable growth solution at a first temperature,and then to institute a cooling program, followed by removal of thesubstrate by simply lifting the slice out of contact with the solution.Unfortunately, a hard crust commonly forms over the surface of the meltwhich frequently prevents efficient removal of the slice. Otherdetrimental effects include uneven growth, etching and breakage of theslice upon removal from the growth melt.

Accordingly, it is an object of the present invention to provide asystem for the epitaxial solution growth of semiconductor crystals whichensures surface protection of the substrate slice prior to immersion inthe growth solution. A further object is to provide a method whichallows removal of the slice at any time increment of the growth orcooling cycle. A further object of the inventin is to ensure completeremoval of gallium solution from the grown surface without damage tothat surface.

Still further, it is an object of the invention to provide an improvedfurnace boat system capable of achieving each of the above objects.

One aspect of the invention is embodied in a furnace boat structurehaving a body member that includes one or more enclosed chambers forholding a suitable melt solution in communication with a slidablesubstrate support member having a recessed area therein for retainingthe semiconductor wafer or other substrate seed". A preferred embodimentincludes means for cleaning the substrate upon sliding the substratesupport member to displace the substrate in contact with, or out ofcontact with, the growth solution.

The structure is basically a closed box arrangement constructed ofgraphite, for example. or other known furnace boat material such assilicon carbide. The slidable substrate support member is preferablydesigned as a push-pull rod which can be displaced in either directionwithout removing the boat from the furnace. The structure is readilyadapted to move a slice or substrate under a saturated growth solution,or more than one growth solution sequentially, or to move a plurality ofslices in contact with a like plurality of separate growth solutions, orthe reverse of any one of these options. It is also possible to retainthe substrate or substrates in a stationary position and to displace oneor more melts in contact with the substrates either simultaneously or insequence.

Thus, it will be apparent to those skilled in the art that the slidingboat ofthe present invention can be employed in the growth of a singlelayer on a single substrate, or the growth of a single layer on manysubstrates, or the growth of sequential layers on a single substrate, orthe growth of sequential layers on a plurality of substrates. It isparticularly advantageous to be able to terminate growth accurately andconclusively at any desired thickness, composition or geometry.

Another aspect of the invention is embodied in a furnace boat structurecomprising a body member having at least one chamber therein for holdinga suitable growth solution, and a slidable substrate support member atthe base of said chamber having a recessed area therein for retaining asubstrate, and means for adjusting the depth of the recessed area forthe accomodation of different substrate thicknesses. A third embodimentof the invention combines the adjustable recess depth of this embodimentwith the cleaning means of the previously described embodiment.

FIG. 1 is a cutaway perspective view, partially in cross-section, of thesliding boat system of the invention.

FIGv 2 is a framentary view, in cross-section, of the boat system ofFIG. 1, wherein the position of the pushrod has been shifted to bringthe substrate in contact with a wiping means, which illustrates a keyfeature of the invention.

In FIG. I the rectangular elongated body structure II is provided withspaced apart, enclosed compartments or chambers 12 and 13 filled withmolten growth solutions 14 and 15 respectively. Graphite felt members 16and 17 are also provided within body member 11 for the purpose ofcleaning the substrate member as it passes underneath. A slideablepush-rod 18 extends the full length of the boat system and includes atleast one recessed substrate support member 19, the depth of which isadjustable by means of screw-threads 20. Support I9 is adjusted toprovide no more than sufficient space to accomodate substrate wafer 21,such that efficient wiping action is assured whenever the push-rod isdisplaced thereunder.

A fragmentary cross-sectional view of the wiping action is shown in FIG.2. It is particularly significant that the graphite felt or othersuitable wiping means is loaded in compression whereby a constant forceis exerted downward thereby increasing the efficiency of the wipingaction.

For example, the sliding boat system of the invention is operated in thefollowing manner. A substrate of a suitable material. including forexample gallium arsenide, gallium arsenide phosphide, gallium phosphide.etc.. is held in the recessed cylindrical space provided by supportmember 19 so that the wafer surface is within a few mils (for example,0.003 inches) of the upper surface of the push-rod 18, which fits snuglyinto grooves or slots milled into the body ofthe boat, which togetherwith the tightly fitting walls thereof ensure little or no verticalmovement of the push-rod. As illustrated in FIG. 1, compartments l2 and13 have a conical shape which contains the appropriate solutions, suchas gallium plus gallium arsenide, plus aluminum and tellurium for growthof n-type gallium aluminum arsenide layers. A similar solution isprepared contain ing zinc instead of tellurium for the growth of p-typelayers.

The tight fit of push-rod l8 effectively seals compartments l2 and 13 toprevent leakage of the liquid melts. The confined space above thesubstrate wafer limits any tendency for the accumulation of gas andthereby resists the amount of arsenic vapor, for example, that canevolve from the surface of a gallium arsenide substrate, therebypreventing decomposition damage to the surface of the substrate in thehigh temperature environment prior to the the growth cycle. Stillfurther, the limited volume restricts the amount of oxygen or otherambient gas which contacts the wafer during the time it is out ofcontact with the growth solutions. Otherwise, an oxide coating wouldform on the substrate thereby preventing optimum growth. Between eachsolution-containing compartment and the central body member are locatedpads of graphite felts l6 and 17 (commercially available material) whosevertical dimension exceeds the heighth of the space provided. Thus, whena lid or other top member is fastened down, compressive force isestablished in the resilient graph ite felt. Quartz micro-fiber,fiber-fax and the like are also suitable. The compression of the padsensures efficient cleaning action.

In operation, the assembly is moved to the center of a uniform hot zonewithin a suitable furnace. A vacuum or inert gas purge or otherappropriate means is used to remove air. Commonly, a pure hydrogenatmosphere is used during the high temperature cycle. After the assemblyhas reached equilibrium at the saturation temperature, the slice ismoved from the center of the system to the far end under the n-typesolution. A controlled temperature decrease is carried out, commonlyover a small temperature span, for example, 0001" to 0.05" per minutefor fifteen minutes to sixty minutes. At the end ofthis period, theslice is moved back to the center which enables graphite pad 16 to wipeoff any remaining gallium solution. Then the slide is moved on fartherto position the slice under the other solution for the growth of ap-type layer. A further controlled temperature program is carried duringwhich time a player 4 is deposited on the preceding n-layer. Note thatgraph ite felt pad 17 provides a second cleaning of the wafer surfaceupon displacement to its position in communication with melt 15.

At the end of the p-growth, the slide is returned to center again,thereby wiping off the surface under pad 17. The assembly is eithercooled in place. or mechanically moved outside the hot zone forimmediate cooling. Note that the newly grown layers are again protectedby the central graphite body which provides spacing that is closer bythe amount of growth. The result is a mirror-smooth, even, planarsurface. The applicability of this invention to continuous processingwill be apparent to those skilled in the art. That is, small individualboats carried by a conveyor belt through a temperature profiled furnacewith semi-automatic mechanical movement of the slide will provide highvolume production.

Multi-layer capability merely requires longer slides and boats.Ultimately, the limitation becomes cost and convenience, instead ofmethod or technique.

It should be apparent that the system of the invention is capable ofproviding epitaxial layers of more accurately controllable thickness,superior flatness, uniformity of surface, and having extremely highcrystal perfection. Additional features include minimum handling ofsubstrates which leads to minimum contamination and minimum cycle time.

What we claim is:

l. A method for the growth of first and second epitaxial layers ofcrystalline material on a suitable substrate comprising the steps of:

a. placing said substrate in contact with a first growth solution atsuitable growth conditions for a time sufficient to deposit a firstepitaxial layer;

b. then transferring the substrate from contact with said first growthsolution. into contact with a second solution for a time sufficient todeposit a sec- Ond epitaxial layer; and

c. concurrently removing substantially all said first solution from thesurface of said first epitaxial layer prior to contact with the secondsolution.

2. A method as in claim 1 wherein said substrate is a lllV semiconductorcompound.

3. A method as in claim 1 wherein said first growth solution comprisesgallium and a Ill-V compound.

4. A method as in claim 3 wherein both solutions comprise gallium plus alll-V compound; wherein said first solution also contains an n-typedopant, and said second solution contains a p-type dopant.

5. A method as in claim 4 wherein said substrate is contained within asliding member of a closed chamber to minimize contact with gaseousambients, and wherein said transfer is achieved by moving said slidingmember in contact with a fibrous pad to clean the substrate surface.

1. A METHOD FOR THE GROWTH OF FIRST AND SECOND EPITAXIAL LAYERS OFCRYSTALLINE MATERIAL ON A SUITABLE SUBSTRATE COMPRISING THE STEPS OF: A.PLACING SAID SUBSTRATE IN CONTACT WITH A FIRST GROWTH SOLUTION ATSUITABLE GROWTH CONDITIONS FOR A TIME SUFFICIENT TO DEPOSIT A FIRSTEPITAXIAL LAYER; B. THEN-TRANSFERRING THE SUBSTRATE FROM CONTACT WITHSAID FIRST GROWTH SOLUTION, INTO CONTACT WITH A SECOND SOLUTION FOR ATIME SUFFICIENT TO DEPOSIT A SECOND EPITAXIAL LAYER; AND C. CONCURRENTLYREMOVING SUBSTANTIALLY ALL SAID FIRST SOLUTION FROM THE SURFACE OF SAIDFIRST EPITAXIAL LAYER PRIOR TO CONTACT WITH THE SECOND SOLUTION.
 2. Amethod as in claim 1 wherein said substrate is a III-V semiconductorcompound.
 3. A method as in claim 1 wherein said first growth solutioncomprises gallium and a III-V compound.
 4. A method as in claim 3wherein both solutions comprise gallium plus a III-V compound; whereinsaid first solution also contains an n-type dopant, and said secondsolution contains a p-type dopant.
 5. A method as in claim 4 whereinsaid substrate is contained within a sliding member of a closed chamberto minimize contact with gaseous ambients, and wherein said transfer isachieved by moving said sliding member in contact with a fibrous pad toclean the substrate surface.